Foreword [Special issue intro.]

The subject of this issue is the integration of microwave circuits-a method for the construction of microwave circuits where photolithographic and chemical processes are the dominant fabrication techniques. This issue is indicative of the variety of integrated microwave circuit development efforts that exist. As reported here, investigations have included hybrid and monolithic circuits; ceramic, semiconductor, and ferrite substrates; distributed and lumped element circuits; special device configurations; computer aided design; and feasibility demonstrations of various circuit classes. In viewing these efforts and their results, it has become apparent that microstrip is quite acceptable for implementing interconnections on a variety of substrates and that circuit losses are not prohibitive; for higher frequency applications (above X band), where surface finish is more critical, single crystal substrates such as semiinsulating gallium arsenide may be required. Further, device packaging has become a limit to circuit experimentation and a demand is being created for characterized unpackaged devices in beam lead, or some equivalent form. Finally, integration has advanced from the realm of novel possibility to near practical reality. Much advanced systems planning is now predicated on the existence of integrated microwave circuits. There is considerable opportunity for innovation with process, device circuit, and application in this area and, as efforts continue to expand, the advantages which have accrued through integration at. lower frequencies should in some measure apply to the microwave area.